The present invention relates to a flow rate sensor capable of conducting measurement of a fluid flow rate with high sensitivity, without the need to heat the fluid to a high temperature.
In a process for manufacturing semiconductors, a flow rate sensor is used as a flow rate controller for stably supplying a liquid material, such as TEOS (tetraethyl orthosilicate), a copper (I) compound, DMAH (dymethyl aluminum hydride) or the like. For use as such a flow rate sensor, in many cases, a thermal type flow rate sensor has been conventionally used. A thermal type flow rate sensor applies heat to a fluid flowing through a sensor tube and detects a quantity of heat carried away by the flowing fluid.
In a conventional thermal type flow rate sensor, a heating temperature for the sensor tube is higher than room temperature by several tens of degrees Celsius. However, some types of liquid materials for semiconductors are extremely unstable relative to heat and form reaction products as a result of heat applied to the sensor tube. Therefore, the sensor tube must be periodically cleaned. Further, as a result of heat applied to the sensor tube, a convection current is generated in air outside the sensor tube, so that a tilt error due to the direction of mounting of the sensor occurs.
Further, when a liquid flows in the sensor tube, air bubbles are likely to be formed in the liquid as a result of heat applied to the sensor tube. The formation of air bubbles can result in large measurement errors.
As a device for heating the sensor tube of the above-mentioned thermal type flow rate sensor, a heating resistor in the form of a wire is wound in a coil around an outer wall surface of the sensor tube so as to provide a resistance of about 100 to 300Ω, and a current is supplied to the heating resistor for heating. The length of the coil is about several mm, and the sensor tube has an inner diameter of about 0.3 mm and an outer diameter of about 0.4 mm. About 100 mW of power is supplied to the heating resistor, so as to heat the heating resistor to about 80° C. A fluid is flowed in the sensor tube in this state, and variation of a resistance of the heating resistor is detected during flow of the fluid (reference is made to, for example, U.S. Pat. No. 3,938,384).
Generally, in thermal type flow rate sensors, an output flow rate from the sensor coincides with an actual flow rate, as long as laminar flow of the fluid can be maintained in the sensor tube. When a velocity of the fluid becomes high, turbulent flow is generated. In this case, the output flow rate becomes lower than the actual flow rate. To enable accurate measurement of a high flow rate of the fluid, a sensor tube having a large diameter and a sufficiently large length should be used. However, such a sensor tube is too large and is unsuitable for practical use. Therefore, conventionally a fluid flow has been divided between a sensor portion in which the flow rate of the fluid is detected in a range of 1 cc/min. or less and a bypass portion for increasing the flow rate. In this technique, no detection of the flow rate is conducted at the bypass portion, so that the bypass can be designed without restraint. Therefore, various arrangements of a small flow rate sensor of this type have been developed.
However, in a flow rate sensor of this type, the inner diameter of the sensor tube is small, so that the flow rate which can be measured by the sensor is limited. In order to measure a flow rate exceeding a certain level, it is necessary to provide a bypass portion in which a fluid flows at a flow rate proportional to that in the sensor portion. This necessitates use of a complicated sensor, and errors arising from the use of a divided flow can result. Further, when the bypass portion is clogged, a large measurement error is likely to occur.